In U.S. Pat. No. 2,749,020, Baxter discloses a bag formed from a thermoplastic film with a transversely extending double seam across the bottom portion of the bag. The double seam consists of a flat seam and an adjoining bead seam along the marginal edge of the flat seam. The method used to form the seam involves passing a flattened tube between a pair of reciprocation heating bars that contact the opposite surfaces of the tube to provide a flat seam transverse across the tube. The tube is then cut adjacent to the cut area, after which the severed edges adjacent to the seam are exposed to radiant heat to melt the edges of the seam, to form a double seam along the bottom of the bag.
In U.S. Pat. No. 3,513,052, Kenji Ariyasu et al. disclose a method for forming a plastic film package comprising a sealed section and beads at the end of the sealed section.
In U.S. Pat. No, 3,684,720, Brady discloses a bag article with a double flat seal along the bottom and side edges. The double seal comprises two spaced apart flat seals.
In U.S. Pat. No. 3,574,039. Fehr et al. disclose a film sealing and cutting system comprising a lower support with a resilient back-up member. A separate hold-down member with spaced parallel holding jaws engages the film to press and hold it against the resilient backup member. A sealing bar suspended between the parallel holding jaws has spaced parallel sealing surfaces arranged to come in contact with the film as it is held against the backup member by the hold-down member. A heated cutting ribbon between the spaced sealing surfaces is moved independently of the sealing bar to cut the film. The sealing bar is not independently heated but is heated by radiation from the cutting ribbon, which a flat blade-like structure.
Howard in U.S. Pat. No. 3,847,711 discloses an apparatus for forming a seal along the edge of a bag with a bead seal along the end of the closure element of the bag. The apparatus comprises a single sealing bar and an opposing pressure bar that move toward each other to engage opposite plastic sheet surfaces. The upper pressure bar has a cutting wire fastened to the middle of the pressure bar so that the cutting wire travels with the pressure bar against the heated sealing bar to cut and seal a film between the pressure and seal bars. The pressure bar incorporates a U-shaped channel for straddling and pinching the closure element, such that a pressure differential occurs to cause a plastic flow into the closure element.
The prior art bags are generally adequate for their designated purpose, but for bags for use in extreme environmental conditions, the seals of the prior bags may be too weak, particularly with "microwave" bags. Microwave bags are intended for the freeze storage of food and also for the cooking of the food in a microwave oven. These bags are often side sealed and have recloseable closure elements near the opening. A requirement is that the closures and seals of these bags remain sealed under the extreme environmental conditions of freezing and cooking. Additionally, if the bags are to be used for "boil-in" cooking by immersing a bags containing a foodstuff in boiling water the bags and closures must be designed to contain moderate pressure.
Bags, in general, can be provided with a bead seal formed by using a heated wire, such as disclosed by Tumminia in U.S. Pat. No. 4,396,449. In this system a heated wire cuts and forms a bead seal. A problem with these bags, particularly in microwave bag applications, is the tendency of the side seals near the bag opening to tear apart. Additionally, bead seals are rarely strong enough alone to contain a moderate steam pressure.
Bags may also be made with a flat seal, such as in the above cited U.S. Pat. No. 3,574,039 and 3,847,711. These seals are stronger, but food storage and cooking bags usually incorporate a reclosable interlocking closure device at the opening, which comprises opposing and mating occludable closure elements. Since the construction of the closure elements often results in open areas between the film sheet sections to be sealed, a flat seal across the closure element is often not completely sealed without the use of extra sealing pressure, a separate sealing step, a stomper bar, or a specially constructed sealing bar. Leakage problems are also caused by the distortion of the closure elements at or near the seal which is caused by prior art methods or which crush the closure elements. The distortion may cause leaks by preventing occlusion of the closure elements near the seal.
Thus, an object of the invention is to provide a thermoplastic bag with leak-free seals of superior strength, particularly bags with reclosable closure elements.